Why does HuntCrossleyForce apply torques?

[Sina Porsa]

Hi,
I just understood that the HuntCrossleyForce contact model applies both forces and torques to the contact sphere, While according to this paper, it should only be a spring and damper force actuator (not a torque actuator).
http://www.stanford.edu/group/nmbl/publ ... rm2011.pdf
When I use the ForceReporter tool, for each contact sphere I'll have 3 forces and 3 torques(in X,Y and Z direction) which is confusing. Am I missing sth?
Thanks
Sina

[Michael Sherman]

Hi, Sina. Hunt & Crossley for a point contact applies just a pure force at the contact point (no torque). Could you be looking at a report of the force that has been shifted to another location? For example, a point force that is applied away from the body origin will show an r X f moment if shifted to the body origin for reporting purposes.

Regards,
Sherm

[Sina Porsa]

Hi Sherman,
I am using a couple of contact spheres on the sole of foot to model the foot-ground contact and obviously the contact spheres are not at foot origin(foot centre of mass).
So this is only reporting the rXf vector an these torques are not applied to the foot? am I right?
Thanks,
Sina

[Michael Sherman]

Yes, the only applied forces are pure forces on the contact spheres, at the point where they touch the ground.

Just to clarify: the contact forces are applied only on the contact spheres. But that could certainly result in a net torque on the foot. For example if only the contact spheres on the left side were in contact, a net torque would result that would tend to rotate the foot to the right. So whether you would say the torque was "applied" to the foot, I don't know. An applied force at a point can certainly generate a moment.

Sherm